Update s6 path in Dockerfile.rpi

Apply the change from https://github.com/gogits/gogs/commit/0cbf56855aad82ff49952132287a0112f308c728 to the Dockerfile.rpi as well

Dockerfile.rpi

@@ -19,4 +19,4 @@ RUN ./docker/build.sh
 VOLUME ["/data"]
 EXPOSE 22 3000
 ENTRYPOINT ["docker/start.sh"]
-CMD ["/usr/bin/s6-svscan", "/app/gogs/docker/s6/"]
+CMD ["/bin/s6-svscan", "/app/gogs/docker/s6/"]